High voltage insulated electrical cable with layer of irradiated semiconductive ethylene copolymer



July 5, 1966 A. N. TQWNE ET AL 3,259,688

HIGH VOLTAGE INSULATED ELECTRICAL CABLE WITH LAYER CF IRRADIATEDSEMICONDUCTIVE ETHYLENE COPOLYMER Filed Jan. 2, 1964 ff? ventons: RobertA. Ward, A//e)? /1 Tow/7e,

y 7776/)" Aorney.

United States Patent This invention relates to electrical conductors andto insulation therefor. More particularly, it relates to such insulatedconductors which are particularly useful for high voltage electricalapplications.

Insulated conductors used for high voltage installations areparticularly subject to corona discharge which occurs at the surfaces ofinsulation or when gas in voids or cavities within the insulation ofsuch conductors is ionized. Such corona leads to deterioration of theinsulation and its eventual failure.

Among the sernicondu-ctive grading insulating media previously used forsuch high voltage conductors were relatively thick elastomeric tapeswound about the conductor. While the electrically conductive gradingmaterial in such installations despite its bulk serves its purpose wherethe tape is actually in good contact with the conductor, the thicknessof the tape, to fulfill strength requirements, inherently produces voidsat points of overlapping, fostering the production of corona anddefeating the purpose of the grading mate-rial. The same deficiencyexists in such thick tapes also when the semiconductive tape is usedover the main ground insulation or next to any outer shield or groundwhich may be present. Among the other types of tapes which have beenused for such semiconducting purposes are the various fabric tapes whichare impregnated with semiconductive materials such as graphite, carbonblack, conductive resins and the like. As in the .case of elastomerictapes, the -fabric tapes inherently produce voids when lapped over theconductor, or when applied as a sheet in the form of a tube. The fabrictapes in general do not have as good adhesion and hence as good contactwith the insulation. The fabric Ibased tapes are also deficient in thatif the ground insulation overcoat is applied by extrusion, the heat andpressure of the extrusion often rupture the cloth-or cause migration ofthe semiconductive particles within the cloth. The prior artsemiconductive tapes and insulations are also subject to the defect thatwhile they can be filled with carbon black or other conductive materialsto a degree, the amount of such conductive filler which can be toleratedis generally limited so that the tailoring of resistivities toparticular installations is often dilficult if not impossible.

From the above, it will be quite apparent that there is a need [forinsulated electrical conductors which are capable of afiording a widerange of resistivities, at the same time providing an insulationstructure which is corona resistant, stable and unitary in character butwhich at the same time can be easily removed for splicing purposes.

A primary object, therefore, of the present invention is to providecables having a composite insulation which is particularly resistant tocorona formation and is possessed of the other above advantages.

Briefly, the invention relates to insulated electrical conductors orcables which are insulated with a first taped or sheet layer ofsemiconducting material comprising copolymers of polyethylene andmono-unsaturated materials as herein more specifically set forth alongwith a ground insulation of extruded polyethylene. In some applications,where indicated, the polyethylene ground in sulation may be surroundedby a suitable second tape or insulation.

3,259,688 Patented July 5, 1966 "ice sheet layer of semiconductive filmand, where indicated, the polyethylene ground insulation may havethereover another layer of polyethylene copoly-mer material as describedabove which is electrically graded in desired manner and over which ametallic shield is wrapped or which is connected suitably to a ground.In some instances, the shield can be directly over the polyethyleneground 'Even when the copolymer outer layer has been finally heat shrunkin place, it can nevertheless still be unwound or stripped for easysplicing.

Those features of the invention which are believed to be novel are setforth with particularity in the claims appended hereto. The inventionwill, however, be better understood and further advantages and objectsthereof appreciated from a consideration of the following descriptionand the drawing in which FIGS. 1 through 3 show various typicalconfigurations, not to scale, of the present insulate-d cables.

The copolymers of polyethylene which are particularly useful inconnection \with the invention are those set forth in copendingapplication S.N. 310,463, file-d September 20, 1963. Among suchmaterials are the copolymers of polyethylene with mono-unsaturatedmaterials such as vinyl group-containing materials, including the vinylesters such as vinyl acetate, the various acrylate materials such as thealkyl acrylates, including methyl, ethyl and hexyl acrylates, amongothers, and mono-unsaturated olefins such as propylene, butenes,pentenes, etc, which are used in such proportions with the ethylene asto produce partially crystalline thermoplastics as opposed toelastomers. It is necessary that these copolymerized materials bemono-unsaturated materials since any higher degree of unsaturationpromotes undesirable cross-linking during preparation of the materialwhich detracts from the flexibility and elasticity and other physicalcharacteristics of the final product. The ethylene and mono-unsaturatedmaterial is copolyrnerized in manners well known to those skilled in theart and such products are commercially available as Union Carbide- DPDB6 l69, DPDA-6'l68, DQDA-3270, DQDA-726 8, DQDA-3269 and Du PontELVAX150 and ELVAX-250, among others.

The above materials are particularly characterized by the fact that theycan accept relatively large quantities of conductive fillers as well asnon-conductive fillers. For example, it has been found that suchirradiated copolymers as those described above will accept up to andover or more by weight of filler having a particle size of from about 10to 20 mp based on the weight of the copolymer without undesirable lossof flexibility or other characteristics which are necessary for asuitable insulating tape. For fillers of larger particle size of from500 to 700 m up to about 400% by weight of filler can be accepted by thematerial without loss of flexibility. Mixtures of such grading materialscan also be used. The filled copolymers which are used as thesemiconducting or electrically grading layer are fully irradiated orirradiated to the point where they are form stable at temperatures of200 C. and upward which are experienced when the polyethylene groundinsulation is extruded over them. They can, of course, be fully orientedor stretched in which case the shrinkage of the tape when the overlayeris extruded on will cause shrinking of the inner layer and heat sealingwhich eliminates any potentially corona-forming voids. Alternatively, asabove, the copolymer grading layer can be irradiated just to the pointof form stability under extrusion temperature conditions and have addedthereto a chemical cross-linking agent such as the various peroxidesdisclosed in British Patent 910,204 and US. Patent 2,916,481, amongothers, which will complete the cure of the material under the extrusiontemperatures polyolefins derived from monoolefins such as polyethyl ene,polybutylene, butyl rubber, ethylene-propylene rubber and blends andcopolymers and terpolymers of such materials which are well known tothose skilled in the art, and where polyethylene is mentioned herein itwill be understood that any and all of such materials are included. Suchpolyethylene materials are sold under the name of Alathon by E. I. duPont de Nemours & Company, Inc., by the Bakelite Company under thedesignations DE- 2400, DYNH, etc., and by the Phillips Petroleum Companyunder the designations Marlex, 20, 50, etc. Butyl rubber is a copolymerof polyisobutylene and isoprene is sold by Enjay Chemical Company as isEPR ethylenepropylene copolymer. The polyolefin ground insulation may beused as such, or it may be cross-linked by irradiation or chemicalcuring agents which are activated by the heat of extrusion. Filledpolyolefin materials can also be used, and among such fillers are thesilica fillers of various sizes and types such as aerogels, fumedsilica, metal silicate, titanium dioxide, zinc oxide carbonates such ascarbamate, magnesium carbonate, carbon black and various carbides.Semiconducting fillers are here used in smaller amounts which will notimpart conductivity to any extent.

The graded or semiconductive materials of the present invention arepreferably provided in the form of tapes or sheets which are typicallyfrom about 2 mils to about 25 mils thick and which are filled withvarying amounts of carbon black or other semiconducting or conductivematerial to provide resistivities for any particular application. Atypical 10 mil thick material has a tensile strength of about 15 lbs.per inch of width, a yield point of 8 to 10 lbs. per inch of width andan ultimate elongation up to about 300%. Resistivities measured at roomtemperature may typically range from about 5 ohm-cm. to 500 ohm-cm. ormore. For example, when a material, specifically Bakelitepolyethylene-ethyl acrylate copolymer DPDB6169, was filled with 25 partsof carbon black per 100 parts of copolymer, the resistivity was found tobe 85 ohm-cm. When 40 parts of carbon black were used, the resistivitywas lowered to 28 ohm-cm. At 60 parts of carbon black the resistivitywas 4.5 ohm-cm. and at 75 parts the resistivity was 1.5 ohm-cm. Theseamounts of filler and even larger amounts up to four times the weight ofthe base material itself are accepted without embrittle- .ment orprohibitive stilfening and are thus distinguished from the normalpolyethylene polymers.

While the grading tape described above can be oriented or stretched andirradiated so that under the heat of extrusion of the ground insulationit will be hrunk about the conductor, it is sometimes preferred tocross-link such tapes by high energy electron irradiation to the pointwhere they are thermoset or form stable and will not rupture under theconditions of heat and pressure experienced during extrusion but at thesame time the irradiation dosage should be low enough so that sealing ofthe tape to itself and to the overlying polyethylene extruded materialis not inhibited. Generally speaking, for the copolymers of the presentinvention radiation levels of 4 to 20 megaroentgens can be used with thepreferable radiation level being between 6 and megaroentgens atpotentials of from 5x10 to 2 10 electron volts. It was found, forexample, that when from 3.5 to 25 megaroentgens were used to irradiate a5 mil thick tapeof the above description, the tensile strength at 150 C.ranged from about 26 to 530 p.s.i. with heat sealing to itself and tooverlying polyethylene occurring at a temperature of 150 C. and apressure of 10 p.s.i.

Any-of the usual well known methods of extrusion can be used. Typically,the temperatures of such extrusion range from about 180 C. to about 260C. and the pressure from about 100 p.s.i. to 1000 p.s.i.

It will be apparent to those skilled in the art that one of the primaryadvantages of the present invention lies in the fact that the relativelythin taped layers which are used herein provide a definite sizeadvantage over the much thicker extruded semiconductive layers of theprior art.

Referring to the drawing, there are shown several e bodiments of thepresent invention, it being realized, of course, that others will occurto those skilled in the art. Shown in FIG. 1 is .an insulated cable 1having a central conductor 2 with a coating of the presentsem-iconductive tape insulation 3 and ground insulation 4 ofpolyethylene type material. Shown in FIG. 2 is an insulated cablestructure 5 with central conductor 6 with semiconductive insulation 7thereover. A ground insulation '8 of polyethylene or polyolefin typematerial as described overlies sem iconductive layer 7. A furthersemiconductive layer '9 is placed about the extruded ground insulationlayer 8. A still further modification of the invention is shown in FIG.3, wherein insulated cable structure 10 having a central conductor 11has a taped. layer of semicond-uotive material 12 and an overlyingextruded layer .of polyethylene ground insulation 13. Grounding shield14 is provided as an outer sheath.

The following examples will illustrate the invention, it beingunderstood that they are not to be taken as limiting in any way.

EXAMPLE 1 A number 2-7 stranded conductor had extruded thereon a layer220 mils thick of uncross-linked polyethylene with no semiconductivelayer. The corona star-ting voltage .of this cable varied from about1,000 to 4,000 volts.

EXAMPLE 2 The cable of Example 1 was tubular wrapped with a layers 5mils thick of copolymer DPDB-6l69 which was filled with 67 parts ofXC-72 Cabot carbon black having a particle size of from about 10 to 20mg, the carbon black being present in the amount of 67 parts by weightper parts of copolymer. Over-lying this semiconductive layer, there wasextruded 220 mil-s of uncross-linked polyethylene. When tested as inExample 1, the corona starting voltage was found to be 35,000 volts.

EXAMPLE 3 A conductor similar to that of Example 1 was overlain with acarbon black filled fabric semiconductive wrap which was in turnoverlain by 220 mils of crosslinked polyethylene. The corona startingvoltage for this cable was found to be about 12,000 volts.

EXAMPLE 4 Example 1 was repeated except that cross-linked polyethylenewas used as the insulation. The corona starting voltage was once againfirom 1,000 to 4,000 vol-ts.

EXAMPLE 5 The conductor of Example 1 was tubular wrapped with asemiconductive layer as in Example 2 and overlain by extrusion withabout 220 mils of cross-linked polyethylone which was in turn overlainwith 5 mils of the present semiconductive tape of Example 2. The coronastarting voltage of this cable was found .to range from about 22,000 to28,000 volts.

There are provided by the present invention insulated cable structureshaving composite insulation, the elements of which are characterized bysimilar coefficient of expansion and which are firmly bound one to theother, thus substantially increasing the corona starting voltage. Theinsulated cables are also characterized by compact size as compared toprior art wholly extruded composite insulations and the semiconductiveelements can be loaded with large amounts of .semicondluctive materialsto provide a wide range of insulated cables which are useful for varyinginstallations. The tape materials of the present invention are alsoparticularly useful in that they may he stripped or sealed to facilitatesplicing where indicated.

What we claim .as new and desire to secure by Letters Patent .of theUnited States is:

1. A high voltage electrical cable having a conductor surrounded by acomposite insulation comprising a first taped layer of irradiatedsemicond-uctive material comprising .a copoly mer of ethylene and amonoaunsaturated material selected from vinyl materials, acrylatematenials :and .olefins and mixtures thereof filled with conductivematerial and a second extruded layer of ground insulation comprising ap-olyiolefin.

2. A high voltage electrical cable as in claim 1 in which said copolymeris a colpolymer vof ethylene and vinyl acetate.

3. A high voltage electrical cable as in claim 1 in which said copolymeris a copolymer of ethylene and alkyl acrylate.

4. A high voltage cable as in claim 1 in which said copolymer :is acopolymer of ethylene and ethyl acry late.

5. A high voltage electrical cable having a centrally located conductorand thereover a composite insulation comprising a first taped layer ofirradiated semiconductive material comprising a cop-olymer of ethyleneand a mono-unsaturated material selected from vinyl materials, iacrylatematerials and olefins and mixtures thereof, filled with semiconductivematerial, .a ground i-nnsnlation of polyolefin extruded over said firsttaped layer, and overlying said ground insulation a second taped layerof similar semiconductive material.

6. A high voltage electrical cable as in claim 5 in which said copolymeris a copolymer of ethylene and vinyl acetate.

7. A high voltage electrical cable as in claim 5 in which said copolymeris a copolymer of ethylene and alkyl .acrylate.

8. A high voltage electrical cable as in claim 5 in 6 which saidcopolymer is a copolymer of ethylene and ethyl a-crylate.

9. A high voltage electrical cable having a centrally located conductorwith .a composite insulation comprising a first taped layer .ofirradiated semiconductive material comprising a *COPlOlYIHfiI .ofethylene and a mono-unsaturated material selected firom vinyl materials,lacrylate materials and olefins and mixtures thereof, filled with con-UNITED STATES PATENTS 2,953,541 9/1960 Pecha et al. 26086.7 X 2,953,5519/1960 White 260-86.7 3,029,230 4/1962 Strauss 260-87.3 3,097,150 7/1963Rainer et a1. 161412 X 3,100,136 8/1963 DAscoli et al 174-102 X FOREIGNPATENTS 450,805 7/ 1936 Great Britain. 861,839 3/1961 Great Britain.

LARAMIE E. ASK-IN, Primary Examiner.

DARRELL L. CLAY, ROBERT K. SCHAEFER,

Examiners.

D. A. KETTLESTRINGS, Assistant Examiner.

1. A HIGH VOLTAGE ELECTRICAL CABLE HAVING A CONDUCTOR SURROUNDED BY ACOMPOSITE INSULATION COMPRISING A FIRST TAPED LAYER OF IRRADIATEDSEMICONDUCTIVE MATERIAL COMPRISING A COPOLYMER OF ETHYLENE AND AMONO-UNSATURATED MATERIAL SELECTED FROM VINYL MATERIALS, ACRYLATEMATERIALS AND OLEFINS AND MIXTURES THEREOF FILLED WITH CONDUCTIVEMATERIAL AND A SECOND EXTRUDED LAYER OF GROUND INSULATION COMPRISING APOLYOLEFIN.